1. Field of the Invention
This invention relates to low pressure turbine apparatus, and, in particular, to a low pressure turbine apparatus having a single casing member.
2. Description of the Prior Art
In the prior art, the typical casing arrangement for a low pressure turbine apparatus comprises a plurality of nested cylinders disposed concentrically about each other. Whether utilized in a single or a double flow turbine, the casing usually includes an inner cylinder No. 1, an inner cylinder No. 2, and an outer cylinder. Each cylinder has, as is well known, a mating cover and base portion.
Inner cylinder No. 1 confines and guides the pressurized motive steam over alternating arrays of rotatable and stationary blading to convert the energy carried thereby into rotating mechanical energy. Inner cylinders No. 1 and No. 2 have as their purpose the isolation of high temperatures and prevention of high temperature gradients, to thus reduce thermal distortions and thermal strains. The nested outer cylinder permits axial expansion of the inner cylinders without affecting the position of the bearing members which support the turbine rotor.
Although the current low pressure casing construction admirably meets all the aforementioned objectives, the disposition of such a large number of major components has a major impact on the cost of the turbine. Presently, there are at least ten major components required in a typical low pressure casing. These elements include an inner cylinder No. 1 cover, an inner cylinder No. 1 base, an inner cylinder No. 2 cover, an inner cylinder No. 2 base, an outer cylinder center section cover, an outer cylinder center section base, and, disposed axially in each side of the center section, an outer cylinder end section cover, and, an outer cylinder end section base. The number of major pieces requiring machining is many. In addition, there is required suitable support and alignment features to permit free movement of the parts due to differential thermal expansion. Further, suitable pressure sealing arrangements must be provided wherever steam inlet connections or steam extracting connections pass through each of the nested cylinders. Since the hotter inner cylinder is exposed to cooler steam on its outside surface, in order to limit thermal gradients, a thermal shield about the innermost inner cylinder No. 1 may also be required.
It is apparent, then that it is desirable to provide a casing for a low pressure turbine having a reduced number of necessary major pieces. As a concomitant to the reduction in number of pieces, the cost of the casing is reduced, due to a reduction in both labor and material costs. At the same time, it is desirable to reduce the number of major pieces, yet maintain reliability and increase the ease of fabrication of the turbine.